1. Field of the Invention
This invention relates to methods of combusting high molecular weight liquid hydrocarbon fuels by co-firing with a more combustible supplemental fuel. More particularly, this invention presents a method and device that effectively combusts heavy hydrocarbon fuel oils by injecting them through a zone of combusting hydrogen where the oil is finely dispersed, partially vaporized and ignited. Since the method presented utilizes a relatively small amount of hydrogen fuel, a low-volume hydrogen source such as the electrolysis of water can be used to generate the required rates of hydrogen. Combustion of heavy oils using hydrogen generated from the electrolysis of water presents a significant achievement over present methods and devices which combust heavy fuel oils by co-firing with large amounts of natural gas. Using the combusting hydrogen to disperse the fuel oil provides the requisite degree of atomization without the need for compressed non-combustible gases, such as steam or air. When used with vegetable oils, the combustion method and device presented herein offers an economical alternative to producing heat energy using only renewable energy sources.
2. The Relevant Technology
Because high-molecular weight, or heavy liquid fuel oils are of such low volatility, a significant amount of heat and mechanical energy must be input to render these fuels into a readily combustible state. Typically, a heavy oil must be heated from ambient temperature to its flash point with even more heat applied to vaporize some of the oil molecules prior to combustion. Co-firing the heavy oil with a readily combustible gas is well known as an effective method of providing the heat load necessary to render the oil to a readily combustible state. Natural gas is presently the most common co-firing fuel since it is highly combustible and often the least costly supplemental fuel source. Natural gas is, however, a non-renewable energy source that may not readily available in some areas and may be subject to other competing domestic and industrial uses.
A majority of present burner designs employ various means of preheating, atomizing and mixing the heavy oil with the hot flue gases from the combusting co-firing fuel to improve heat transfer. Fuel atomization increases the exposed surface area of the liquid fuel, which increases the rate of vaporization. Three primary means are employed for atomizing the liquid fuel: 1) liquid feed nozzles, 2) high-pressure steam or air-assisted jetting, or 3) rotating cups. Examples of these atomizing methods include Pressure Jet Atomizers and, Steam or Air Assisted Jet Atomizers and Low pressure Air Atomizers. The Pressure Jet Atomizer utilizes high oil feed pressure to atomize the fuel into a spray of finely dispersed droplets. The fuel oil is fed into a swirl chamber by means of tangential ports in the main atomizer body. An air core is set up due to the vortex formed in the swirl chamber, which results in the fuel leaving the final orifice as a thin annular film. The angular and axial velocity of this film causes the fuel to develop into a hollow cone as it discharges from the orifice. One major problem with these types of burners is that the atomizer has a distorted spray angle as the fuel flow rates are reduced, which often results in fuel/flame impingement on the furnace walls.
The External-mix Steam Atomizer or Steam-assisted Pressure Jet Atomizer type burners are designed to make full use of pressure jet atomization at high firing rates and blast atomization at low firing rates. The external-mix style employs an atomizer with a pressure jet tip, around which is provided a steam supply channel. The steam exits this annular passage way through a gap at an angle and swirl that substantially matches the oil-spray cone angle. Since the fuel oil and steam are not pre-mixed, the output is unaffected by slight variations in the steam pressure. An alternate method is the internal-mix steam atomizer, which is comprised of two concentric tubes, a one-piece nozzle and a sealing nut. The steam is supplied through the center tube and the fuel oil through the outer tube. The outlet of the center steam tube has a number of discharge nozzles arranged on a pitch circle such that each oil bore meets a corresponding steam bore in a point of intersection. At the steam exits these nozzles, it mixes with the oil forming an emulsion of oil and steam at high pressure. The expansion of this mixture as it issues from the final orifice produces a spray of finely atomized oil.
The Rotary Cup atomizer employs a cup-shaped member that rotates at high speeds (around 5000 RPM) by an electric motor and belt drive. The fuel oil flows at low pressure into the conical spinning cup where it distributes uniformly on the inner surface and is spun off the cup rim as a very fine oil film. A primary air fan discharges air concentrically around the cup, striking the oil film at high velocity and atomizing it into tiny droplets. The rotary cup burner has good turn down ratio and is relatively insensitive to contaminants in the fuel oil. The Low-Pressure Air Atomizer employs a principle is similar to that of the rotary-cup-atomizing, but the liquid fuel is forced to rotate in a fixed cup by means of a forcefully rotating primary airflow.
Although the aforementioned burners are typically designed to combust lighter fuel oils, such as diesel fuel, they must be modified to combust heavier fuel oils. Typical modifications include equipping the combustion chamber or the area around the oil filming/atomizing device with a plurality of ports where a natural gas can be fed to the combustion zone. The natural gas is ignited first and the oil flow is started once a stable gas flame is established. As the molecular weight of the fuel oil increases, the amount of natural gas required to completely combust the oil also increases. Although natural gas is presently the most common co-firing fuel, the amount required to thoroughly combust a heavy oil can be substantial.
Hydrogen is generally known to be an improved co-firing fuel primarily because its heat of combustion and adiabatic flame temperature are much higher than methane, the primary constituent of natural gas (61,100 btu/ft3 versus 23,879 btu/lb on a gross basis, 3,861° F. versus 3,371° F.). For a typical direct co-firing burner, more than 2.5 times as much natural gas would be theoretically required to produce the same amount of heat as a given mass of combusting hydrogen. Also, hydrogen is further preferred over natural gas because it can be generated from renewable energy resources and its combustion product, water vapor, is more friendly to the environment. However, simply replacing natural gas with hydrogen is not generally feasible because even 2.5 times less gas rate would still constitute a significant hydrogen demand for a standard industrial-sized burner and methods do not presently exist that can economically generate and store large volumes of hydrogen for such an application.
Although the potential benefits of using hydrogen as a co-firing fuel are generally known, the practical difficulties of handling and combusting hydrogen have largely prevented the development of useful combustion devices employing hydrogen as a co-firing fuel. Hydrogen's extreme combustibility makes its generation, storage and handling expensive and potentially dangerous. Secondly, hydrogen's flame velocity is more than 8 times as fast as a typical heavy fuel oil flame velocity. This characteristic makes co-firing by conventional burners largely ineffective because the hydrogen burn rate substantially outpaces the fuel oil burn rate and the flame propagation may not be stable without a large excess of hydrogen.